This invention relates to an image forming apparatus such as a printer, and particularly relates to an image forming apparatus having a developing device that uses a developer such as a toner.
Generally, an electrophotographic image forming process includes a charging process, an exposing process, a developing process, a transfer process and a fixing process. In the charging process, a photoconductive insulation layer of a latent image bearing body is uniformly charged. In the exposing process, the photoconductive insulation layer is exposed with light, and electric charge on the exposed portion vanishes so that a latent image is formed. In the developing process, a toner as a developer (including, at least, a coloring agent) adheres to the latent image and visualizes the latent image. In the transfer process, the visualized image is transferred to a printing medium such as a transfer paper or the like. In the fixing process, the transferred image is fixed to the printing medium by means of heat and pressure, or other appropriate fixing method.
In an image forming apparatus using the above described electrophotography, the density unevenness may appear in the form of stripe pattern on the image (referred to as image jitter) due to uneven movement of mechanical components (for example, uneven rotation of the latent image bearing body), and therefore the image quality may be degraded. The uneven rotation of the latent image bearing body is caused by a mechanical load or the like applied by a developer bearing body disposed in contact with the latent image bearing body. Japanese Laid-Open Patent Publication (see Page 1 and FIG. 1) discloses the image forming apparatus in which the circumferential (linear) speed (i.e., printing speed) of the latent image bearing body is set within a range from 20 to 150 mm/sec and the toner cohesion is defined within a range from 30 to 80%. With such conditions, the load applied to the latent image bearing body by the developer bearing body can be reduced, and the image jitter can be restricted.
However, recently, the printing speed of the image forming apparatus becomes faster in order to meet the demand for high speed printing. As the printing speed becomes faster, the toner tends to be compressed in the vicinity of a contact portion between the developer bearing body and a developer layer forming member (such as a developing blade) for forming a toner layer on the developer bearing body, even if the toner cohesion is defined as disclosed in the above described publication. This phenomenon is called as “packing”. In such a case, the developer bearing body is applied with a pressure due to the packing of the toner, in addition to a pressure applied by the developing blade under ordinary conditions. Consequently, at a portion where the packing of the toner occurs, the pressure applied to the developer bearing body may increase, and the amount of the toner may decrease. As a result, the thickness of the toner layer on the developer bearing body may become uneven, and therefore the image jitter may appear on the image, with the result that the image quality may be degraded.